early every afternoon, onshore ocean breezes push pollution against the coastal ranges around San Diego. Emissions from motor vehicles and industry are generated in the populated coastal plain and blown inland by the onshore breezes to the lower mountain slopes.

At night, the process reverses, taking it back towards the ocean. The next day, the cycle begins again and continues until weather patterns change. This collection of trapped pollutants pushes against mountain slopes on a daily basis and in the intense sunlight react together to create ozone, the main ingredient of smog. This photochemical process peaks in the afternoon, when sun is most abundant, and smog hovers around 2,000 feet.

When conditions are right, a temperature inversion layer, or a kind of lid, is formed when warm, dry air overlies cool, moist marine air. The layers cannot mix. The inversion layer traps pollutants against the slopes and prevents them from rising over the mountains. This inversion layer traps both local and transported dirty air.

Organized wind patterns in the summer cause "Santa Ana" weather conditions. Winds blowing toward the southwest transport polluted air from the South Coast Air Basin (the
metropolitan areas of Los Angeles, Orange, San Bernardino, and Riverside Counties) out over the ocean, and the sea breeze brings it onshore into San Diego County. The winds create an eddy or swirl-like pattern that circulates around the coastal area and is blown farther inland.

The typical wind flow pattern fluctuates with occasional winter storms. In the winter, the winds are slower and don't blow in a prevailing direction so there is no eddy pattern. But because of the low angle of the sun, inversions hang around even longer trapping pollution until the weather changes.

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Eye-opening Fact...
Because of unique topography, a large population and nearly one car per person, California still has eight of the 10 smoggiest cities in the U.S.

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